Until very recently, the primary flight control systems of commercial aircraft have utilized mechanical cables to transmit pilot produced control inputs to the control surfaces of the aircraft. Pilot-produced inputs are created by a pilot moving various pitch, roll and yaw axis control devices, such as the column, wheel and rudder pedals located in front of the pilot (and co-pilot) seat(s) in an aircraft cockpit. The control surfaces of the aircraft include the elevators, ailerons, spoilers and rudder of the aircraft. In operation, a pilot manually "flies" an aircraft by moving various pitch, roll and yaw axis control devices so as to position the control surfaces in a way that makes the aircraft follow a desired flight path through space. Aircraft weight, center of gravity location, aerodynamic configuration and location in a flight envelope determine how the pilot positions the pitch, roll and yaw axis control devices in order to follow a desired flight path. Changes in any of these factors require that the position of the control devices be different, even when performing the same maneuver. Especially in adverse weather conditions (e.g., turbulence, wind shear, precipitation and poor visibility), pilot workload from manually operating pitch, roll and yaw axis control devices, plus navigation and other equipment, can become excessively high. Excessive pilot workload has the possibility of compromising safety, particularly during critical portions of a flight, such as approach, flare and landing.
Over the years, flight control system improvements, such as hydraulically powered control surfaces, "feel" systems, ratio changers and yaw dampers, have helped to reduce pilot workload and to provide a more uniform airplane response to given pilot control inputs when an aircraft is operating with different center of gravity locations and in different parts of its flight envelope. While these improvements have helped to reduce pilot workload under normal operating conditions, they have only partially solved the pilot workload problem in adverse weather conditions. The pilot workload problem in adverse weather conditions has been only partially solved because, even with these improvements, the basic way a pilot flies an airplane has remained unchanged--the pilot still commands control surface positions by positioning pitch, roll and yaw axis control devices.
The generation of commercial transport aircraft presently being developed will feature electronic flight control systems that are expected to provide a quantum reduction in pilot workload and a quantum improvement in flying qualities. Electronic flight control systems, such as fly-by-wire (FBW) and fly-by-light (FBL) flight control systems, will permit a pilot to command parameters other than control surface position with available control devices. For example, one system presently being considered for use on the next generation of Boeing aircraft allows a pilot to command flight path angle rate-of-change through a pitch-axis control device. The pitch-axis control device is presently contemplated to be in the form of either a conventional control wheel column or a sidestick controller. Based on the pilot's positioning of the control device, the electronic flight control system will command the elevators of the aircraft to move in the manner required for the aircraft's actual flight path angle rate-of-change to follow the pilot's commanded flight path angle rate-of-change. In other words, a given input on the pitch axis control device will command a given rate-of-change of the flight path angle of the aircraft. Returning the control device to a neutral position will zero the rate-of-change, not the flight path angle. As a result, when the control device is placed in a neutral position, the aircraft will maintain the previously set flight path angle. Changing from a climb angle (or a descent angle) to level flight will require that a pilot move the control device to cause the flight path angle to decrease (or increase) until the aircraft's flight path is level. That is, movement of the pitch axis control device will cause a flight path angle change to occur that will ultimately result in the aircraft reaching level flight, at which time the control device will be moved to its neutral position. The magnitude of pitch axis control device movement will control the magnitude of the rate-of-change of flight path angle.
The foregoing system reduces pilot workload because the electronic flight control system forces the aircraft to follow the pilot's flight path angle rate-of-change commands regardless of aircraft inertia or aerodynamic configuration, location in the flight envelope, or the presence of external disturbances such as turbulence and wind shear. While a flight path angle rate-of-change electronic flight control system is expected to considerably improve pitch-axis flying qualities during the takeoff, climb, cruise and descent portions of a flight, the use of flight path angle rate-of-change commands during the flare portion of a landing poses problems in two specific areas. First, in a conventional flare maneuver, a pilot pulls back a control column or stick and holds it back until the wheels of the aircraft touch the runway, i.e., the aircraft lands. This action causes a flight path angle change that is dependent on how far the stick is pulled back and held. In a flight path angle rate-of-change electronic flight control system, a flight path angle change is caused by applying a control pulse, not a steady pull and hold, to a pitch axis control device. This procedural difference requires pilot retraining and the additional expense associated with such retraining in order for a flight path angle rate-of-change electronic flight control system of the type described above to be used during the flare portion of a landing. The second problem area relates to the risk of overflaring. The risk of overflaring is increased substantially using a flight path angle rate-of-change electronic flight control system if a pilot does not return the pitch axis control device to its detent (i.e, neutral) position at the precise time the flight path angle of the aircraft achieves the small negative number, approximately -0.5.degree., that produces a low-sink-rate landing. If the flight path angle goes positive, a pilot using a flight path angle rate-of-change electronic flight control system is required to push the pitch axis control device to a position that creates the negative flight path angle rate required to restore the negative flight path angle needed to continue the landing. In summary, unconventional pilot maneuvering techniques during the flare portion of a landing are required with a flight path angle rate-of-change electronic flight control system of the type described above.
One obvious way to avoid the foregoing problem is to deactivate the flight path angle rate-of-change electronic flight control system during the flare portion of a landing. This approach has the disadvantage of losing all of the turbulence rejection, configuration masking and flight-envelope location masking effects normally provided by an electronic flight control system. Because flare is a high pilot workload portion of a flight, the loss of handling qualities and turbulence rejection benefits during landing flare is highly undesirable.
The present invention is directed to avoiding the foregoing and other problems by providing a flare control modification for the pitch axis part of a maneuver command electronic flight control system that maintains the advantages of the system while allowing the pilot to use conventional piloting techniques during the flare portion of a landing. More specifically, the present invention is directed to providing a flight path angle command flight control system for landing flare that reduces the risk of overflaring during the flare portion of a landing maneuver while retaining the turbulence rejection, configuration masking and location in the flight envelope masking benefits provided by closed loop electronic flight control systems.